You can write both examination. However, by experience, AAT is much more easiler then ACCA. In addition, HKAAT has a role to go HKICPA, but ACCA do not. You have to go both a degree and ACCA before you can be qualified in CPA. AAT is the UK qualification and HKAAT is the local qualification. To be HK CPA, you have to write HKAAT.